Amplitude- and frequency-modulation radio receiver



C. T. M COY Aug. 1, 1944.

AMPLITUDE-AND FREQUENCY-MODULATION RADIO RECEIVER I 2 Sheets-Sheet 1 Filed NOV. 14, 1941 #52; zaassLy- 4 COUPLED 7'0 WIND/N0 8.

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Filed Nov. 14, 1941 2 Sheets-Sheet 2 All An vIvyIII tug k EM 3 tkgb Elohim 3% Numb .Snk SQQ 3 Patented Aug. 1, 1944 AMPLITUDEl- AND FREQUENCY-MODULA- TION RADIO RECEIVER Claudius McCoy, Philadelphia, Pa., assignor to Philco Radio and Television Corporation. Philadelphia, Pa, a corporation of Delaware Application November 14, 1941, Serial No. 419,200

Claims. 250) This invention relates to radio receivers and to radio receiver circuits adapted for use in the reception of amplitude-modulated and frequencymodulated carrier waves. More particularly, the invention relates to certain novel .circuit arrange ments for AM/FM receivers which do not require switching when changing fromAM to FM opera tion, or vice versa.' I

Radio receiver are known which include separate amplitude-modulation and frequency-modu lation channels, with means for switching at'will from one channel to the other. Such receivers are effectively a combination, within a common cabinet or housing, of a complete FM receiver and a complete AM receiver, having only the power supply system, theaudi'o amplifier, and the loudspeaker in common' This construction results in an uneconomical duplication of the radio frequency, first detector, intermediate fre quency. and second detector circuits and tubes, and in addition greatly complicates the marine" facture of the apparatus. g

The principal object of this invention isto provide an improved and economical combination AM/FM radio receiver.

Another object of this invention is greatlyto simplify the construction 'of combination AM/FM receivers, and to utilize all or most of the tubes during both AM and FM operation, therebyef fecting a' substantial economy in tubes [and apparatus. V i

Another object of the invention "is to provide a combination AM/FM receiver in which the number of individual switching operations required in changing from the FM to the AM bands (or vice versa) is held to a minimum.

Still another objectof the invention is to provide a balanced frequency-detector so constructed and arranged that it can also be employed as an amplitude-modulation detector without switching the detector circuits themselves.

A further object of the invention is to provide a selective intermediate frequency amplifier network capable of amplifying and transferring, without switchingor with a minimum of switch ing. either a frequency-modulated wave of relatively high intermediate frequency, or an amplitilde-modulated wave of lower intermediate frequency. 7 Y Other objects and features of the invention will be apparent from the followng description and. the accompanying drawingain which:

Fig. 1 is a schematic representation of a prior art balanced: frequency-detector, which is illustrated to enable the operation and application of the invention to be better understood and appIeciated;

. Fig. 2 is a schematic diagram of one embodi ment of the switchless AM and FM detector of the present invention; Fig. 3 is a schematic diagram of a preferred em: bodiment of the switchless AM and FM detector; Fig. 4 is a perspective view of a detector input transformer suitable for use with the detector cir cuits illustrated in Figs. 2, 3and 5; Fig. 5 is a schematic diagram of a radio receiver embodying the switchless AM/FM detector and a switchless AM/FM intermediate frequency am plifier network; and

Fig. 6 is a persepctive view of an intermediate frequency transformer adapted for use in the radio receiver of Fig. 5. Attention is now directed to Fig. 1; wherein there is shown, by way of illustration, a balanced frequency-detector of a type to which my invention may advantageously be applied, as will be explained hereinafter in connection with Figs; 2, 3, and 5. The detector of Fig. 1 is derived from the balanced frequency detector described and illustrated in German Patent No. 428,643 granted May 14, 1926. This detector is constructed, and operates, generally as follows. A pair of diodes D1 and D2, or other suitable rectifier elements, are provided, repectively, with the untuned 'in-= put windings l and 2, thewinding, I being con nected to the anode of the diode D1, while' the winding 2 is connected to the anode of the diode D2. The tuned winding}, on the other hand, disposed in a circuit which is common to both diodes; the winding'ib'eing connectedbetween the junction ofthe untuned coils I,'2, and the rectifier cathodes, the latter connection being effected by'means of the radio frequency by-pass condensers 4 and 5. The diode D1 is provided.

with a load resistor B and the diodefDz' with a load resistor 1, these load resistors being connected in shunt with the condensers 4 and 5, respectively. The frequency modulated signalmaybe supplied to the detector circuit by way of the tuned input winding 8. which, in a superheterodyne receiver, may be the output circuit of the receivers intermediate frequency amplifien Preferably, the untuned windings l and 2 are tightly coupled to the winding 8, while the'tuned wind;- ing 3 is loosely coupled thereto. The windings I and 2 are wound in a direction such that the voltages at the opposite ends thereof are substantially -l degrees out'cf phase with each other. The tuned wind ngs 3 and 8 are preferably tuned to the center-frequency of the irequency-modulated wave supplied by the preceding I. F. amplifier (not shown), and when so tuned an unmodulated input signal of this frequency will give rise to voltages across the untuned windings I and 2 which differ in phase from the voltage across the tuned winding 3 by substantially 90 degrees. Consequently, the d ode D1 will be supplied with a signal voltage which is the vector sum of the voltages across the windings l and 3, while the diode D2 will be supplied with a signal voltage which is the vector sum of the voltages across the wind ngs 2 and 3. At resonance the vector sum of the voltages applied to the diode D1 will equal the vector sum of the voltages applied to the diode D2, and, hence, for this condition the rectified output signals appearing across the load resistors 6 and 1 will be equal, and, with the differential connection shown, opposite. As the frequency of the incoming signal varies, the phase of the voltage across the tuned winding 3 will change with respect to the voltages across the untuned wind-v ings I and 2, and in consequence the diodes D1 and D2 will be supplied with input voltages differing both in phase and magnitude. Thus, when the signal frequency is above resonance, the voltage applied to one of the diodes will be greater than that applied to the other diode, while when the signal frequency is below resonance, the converse will be true. In a detector of this type, the input circuit for the diodes, comprising windings l, 2 and 3, is a discriminator type circuit because it discriminates between signal frequencies above and below the center frequency to which the winding 3 is tuned.

The difierential connection of the load resistors 6 and 1 permits the derivation from the cathode of D1, of a detected output voltage whose magnitude is proportional to the difierence between the signal frequency and the resonant fre quency of the tuned winding 3. As is well known, the differential connection of the load resistors B and 1 makes the detector highly sensitive to carrier frequency changes, but very insensitive to carrier amplitude changes, and, hence, this detector is relatively insensitive to noise;

A more detailed description of the operation of this form of detector is deemed unnecessary, since the basic theory of operation is adequately treated in the above-mentioned German patent.

In Fig. 2, there is illustrated one embodiment of the detector provided by the present invention, which utilizes the principles of the prior art detector of Fig. 1. Certain elements in Fig. 2 correspond to the elements of Fig. 1 andare correspondingly designated. The device of Fig. 2 difiers from that of Fig. 1 in that the device of Fig. '2 is capable of detecting both ampl tudeand frequency-modulated signals, and this dual function is accomplished, in accordance with this invention, without detector-circuit switching. As in Fig. 1 the untuned windings I and 2 are tightly coupled to the primary winding 8, while the tuned winding 3, which is common to the input circuits of both diodes, is loosely coupled to the Winding 8. The added elements, which comprisethe tuned and coupled w ndings 9 and I0, make it possible to utilize certain of the elements which are common to Fig. 1 for detection in both the FM and AM bands. As in Fig. 1, the windings 3 and 8 may be tuned to the intermediate frequency employed in FM reception, commonly 4.3 megacycles, while the windings 9 and I may be tuned to the intermediate frequency employed in AM reception, commonly 455 kilocycles. As is disclosed in detail in Fig. 5, the radio frequency and intermediate frequency stages of the preceding receiving circuits may be constructed and arranged to supply either a frequency-modulated 4.3 megacycle signal or an amplitude-modulated 455 kilocycle signal to the detector input terminal I l. Of the former signal is supplied, the signal will be applied to the diodes by way of the windings 8, l, 2 and 3, no substantial voltage appearing across the windings 9 and [0 because they are tuned to a frequency differing substantially from that of the signal frequency. By so designing the tuned transformer 9, ID, that the impedance of the condenser across the winding I9 is negligible at 4.3 megacycles, the series insertion of this tuned circuit between the upper end of winding 2 and the lower end of Winding I will have no appreciable efiect upon the operation of the circuit as a detector of the frequency-modulated 4.3 megacycle signal, the circuit functioning substantial?y as described with reference to Fig. 1.

If now an amplitude-modulated 455 kilocycle signal be supplied to the terminals H, the operationabove outlined will be relatively reversed, i. e., no substantial signal will appear across the windings 8, I, 2, or 3, but instead signal will appear across the 455 kilocycle windings S and I 0. Moreover, at this relatively low frequency (455 k.- 0.), the impedance of the 4.3 megacycle windings 8, I, 2, and 3 will be negligibly small, and, hence, these windings will exert no appreciable effect upon the operation of the circuit. The amplitude-modulated 455 kilocycle signal appearing across the tuned secondary winding H3, is applied to the diode D2 as follows-from the lower terminal of the winding ID, through the winding 3 (having negligible impedance of 455 k. 0.), and through the I. F. by-pass condenser 5, to the cathode of D2 and chass s (ground); and from the upper terminal of the winding Iii, through the winding 2 (of negligible impedance at 455 k. c.) to the anode of D2. No appreciable 455 kilocycle signal is applied to the diode D1, since both its anode and cathode are effectively grounded for 455 kilocycle currents, and, hence, this diode will have no substantial effect upon the 455-kilocycle operation of the circuit. The audio output voltage is, of course, developed across the load resistor l, and may be supplied to the following audio amplifier by way of the resistor 6, as will be shown in greater detail in Fig. 5.

It should be understood that the invention herein disclosed is not limited in its application to balanced detector circuits of the type illustrated in Fig. 1, but is capable of application to balanced detector circuits generally. Other known balanced detector circuits to which the invention may be applied are, for example those disclosed in the patents to Round and to Seeley, Nos. 1,642,173 and 2,121,103, respectively. The application of the invention to these and other detectors will be obvious to those skilled in the art, and specific illustration thereof is deemed unnecessary.

Attention is now directed to Fig. 3 wherein is illustrated a combination of the above-described switchless AM/FM detector circuit and the novel AVC. circuit described and claimed in copending application, Serial No. 415,465, filed October 17, 1941, Patent No. 2,330,902, October 5, 1943. The circuit of Fig. 3 differs from that of Fig. 2 in the interconnection and utilization of its load resistors 6 and 1. The differential connection between the resistors is maintained, for audio frequencies, by the connection of an audio by-pass condenser l2 between the lefthand (anode) ends of the said resistors. However, for direct currents, the resistors are 'con-' nected 'in a relatively reversed fashion' by means of a high resistance element I 3. The latter element, which may be a one-megohm resistonmay be connected between the right-hand'icathode) end of the resistor 6, and the left-hand (anode) end of the resistor 1. This connection is such that, during the reception of frequency-m'odu lated signals, the direct-current products of the rectification process are added to producean overall direct-current output voltage which is substantially double that established across "either resistor alone. This additively-combined direct-current output voltage may be filteredby means of the filter network l4, I5, and employed to control the gain of a preceding radioor intermediate frequency amplifier stage. As is de-' scribed in'detail in the above-mentioned copending application, the advantages of this novel AVC connection he not only in increased AVC voltage output, but also in the fact that the magnitude of the AVG voltage varies symmetrically with the tuning of the receiver about the proper tuning point.

the reception of amplitude-modulated signals, under which condition only the diode D2 is supplied with signal voltage, AVC voltage may be derived from the negative (anode) end of the load resistor I by way of the serially-connected resistors, l3, 6, and M, the control voltage appearing at the junction of the resistor l4 and the condenser 15. The audio output signal may again be derived from the junction of the resistorsii and I3 and the cathode of D1, this point being coupled to the ungrounded end of the resistor I through the resistors 6 and I3, which are effectively in parallel for audio, fre quencies since the impedance of the condenser I2 is negligible at these frequencies. 7

Reference may now be had to Fig. ,'which illustrates a detector input transformer of a type suitable for use with the dual detectors of Figs. 2, 3, and 5. This'transformer may comprise an insulating tube 16 fixed to an insulating support [1, the latter carrying, as is conventional, the several condensers and trimmers necessary for properly tuning the windings carried by the tube 16. The windings l, 2, 3, 8, 9, and I correspond, respectively, to the likenumbered windings of Figs. 2, 3, and 5, and in one embodiment of the invention had inductances of 37, 3'7, 33, 39, 200, and 200 microhenries, respectively. The windings 9 and H! are preferably shunted by large enough tuning capacities to bypass 4.3 megacycle currents, and are preferably tightly coupled to give two resonant peaks. These windings, in the preferred embodiment, are thentuned to give one peak at 455 kilocycles '(or whatever the selected I. F. happens to be), the frequency of the other peak being made high enough, by the tightness of coupling, not to affect the adjacent carrier attenuation at 455 kilocycles. This method of construction enables the 455 kilocycle transformer to be tuned by the adjacent of a single trimmer condenser, e. g. one on the primary side, as indicated in Fig. 5.

- The windings I, 2, and 8 may be grouped well together, as shown, to get the desired tightness of coupling therebetween, the untuned windings I and 2 being, positioned symmetrically on either side of the tuned primary winding 8. The tuned secondary winding 3 may be placed at some point on the tube l6 remote from the tuned pri-" mary winding 8 to secure the desired loose ecu-'- pling between these tuned windings. The entire assembly is preferably housed in a conventional'shield can to reduce the coupling between its windings and other components of the radio receiver in which-the device is employed. l

A very slightly modified version of the switchless AM/FM detector circuit, together with the novel switchess AM/FM intermediate frequency coupling network (not yet described) is illustrated in Fig. 5, which is a schematic diagram of a substantially complete AM/FM radio receiver of the superheterodyne type. In general, this receiver comprises a loop antenna |8 for the reception of amplitude-modulated signals, e. g., in the broadcast band, 550 to 1600 kilocycles; a dipole antenna l9 forthe reception of frequency-modulated signals in the FM band, 42 to 50 megacycles; a local oscillator tube V1, a first detector tube V2, a pair of intermediate frequency amplifier tubes V3 and V4, and a detector; audioamplifier, and AVC tubevs A series of ganged switches, S1 to S may be provided for switching various of the circuits to condition the receiver for reception of signals either in the AM or in the FM band. If additional bands are desired, e. g., one or more short-wave bands, they can readily be added in accordance with the conventional practice. In Fig. 5 the switches are shown in their FM positions, switches S1 to S3 being connected in the antenna or R. F. circuits, S4 to S7 in the oscillator circuits, S8 and S9 in the first I. F. circuit, and S10 in the grid bias cir- 'cu t of the first detector tube V2.

With the switches in the FM position, H the short-wave dipole I9 is coupled to the input circuit of the first detector tube V2 through the transmission line 20, the matching transformer 2|, the switch S1, and the conductor 22. The secondary of the transformer 2 i may be tuned by one unit C2 of a ganged tuning condenser 81$.- ment. Inductively coupled to the transformer 2| is the oscillator transformer 23 having a plate winding 24 tuned by the variable condenserv C1, and an untuned grid winding 25. The latter winding is connected to the grid of the oscillator tube V1 by way of the conductor 26 and thec'on denser 21, while the plate winding 24 is connected to the plate electrode of V1 by way of the conductor 28, the switch S5, and the condenser In the FM position of the switches, the ire:- quency-modulated I. F. signal output ofth first detector V2 is applied to the grid circuit of the first I. F. amplifier tubeVs by way of an I. F. coupling transformer 30. With the switch S3 in the FM position, the primary side of this trans former comprises the winding 3| tuned by the condensers 32 and 33 in series. The damping resistor 34 is effectively connected in shunt with the condenser 33,'the impedanceof condenser 35 being negligible at the intermediate frequency. The damping resistor serves to'fiatten the re sponse characteristic of the network, and is conventional in FM receivers. With the switch S9 in the FM position, the secondary side of transformer 30 comprises the winding 36 tuned by the condensers 31 and 38 in series, the winding 39 being short-circuited by the switch ss.

The I. F. coupling transformer 4!! is capable of transferring signals of either the AM or the FM intermediate frequencies, from the output of: the first I. F. amplifier tube V3 to the input of the second I. F. amplifier tube V4. The novel construction of this coupling element will be dee scribed in some detail hereinafter;

The detector circuit which is coupled to the output of the second I. F. amplifier tube V4 is substantially identical to that illustrated in Fig. 3, and the corresponding elements have been correspondingly numbered to simplify comparison of the circuits. In Fig. 5, the audio bypass condenser I2 is connected between the lower end of coil llland the left hand (anode) end of the resistor 6, rather than between the left-hand ends of the resistors 6 and I as in Fig. 3. Likewise, in Fig. 5, the upper terminal of the coil 2 is connected directly to the junction of resistors 1 and I3, while in Fig. 3 the winding i is interposed in this connection. Since the impedance of the tuned circuit II] is negligible at the FM intermediate frequency, it is obvious that these difierences of wiring will have no substantial effect during FM operation of the receiver. During AM operation, however, the connection of Fig. 5 places the diode load resistor in shunt with the tuned secondary winding l0, rather than in series therewith, as is the case in the connection of Fig. 3. Thus, in the circuit of Fig. 3, the effective Q of the tuned secondary winding will be higher than in the circuit of Fig. 5,

in which latter case the tuned circuit is damped by the application thereacross of the load resistor 1.

In Fig. 5, as in Fig. 3, the audio output signal may be derived from the cathode of the diode D1, i. e., from the junction of the resistors 6 and I3. The signal from this point may be supplied to a suitable Volume control potentiometer M by wayof an I. F. filter network comprising the series resistor 42 and I. F. by-pass condenser 43, and the coupling condenser 44. If desired, the volume control may be provided with a conventional treble compensating condenser 45, and a variable bass compensating. network 46, 41,48. The audio signal taken from the potentiometer 4| may be applied, by way of the. condenser 49 A and conductor 50, to the control grid of the triode section of the vacuum tube V5. Additional I. F. filtering may be secured by connecting an I. F. by-pass condenser 5| in shunt with the output circuit of V5, as shown. Similarly, a conventional variable-high-cut tone control, comprising the condenser 52 and the variable .resistance device 53, may be connected between the triode anode and ground.

In the AM operation of the radio receiver of Fig. 5, the switches S1 to S are turned to their alternative positions, the receiver thus being conditioned to receive amplitude-modulated signals in a predetermined AM band, e. g. the broadcast band. Under these conditions, the FM dipole circuit is opened, and the loop antenna l 8 is connected to the input circuit of the first detector tube V2 by way of the conductors 54, the matching transformer 55, the switch S1 and the conductor 22. The tuning condenser C3 is connected in shunt with a portion of the matching transformer 55 through switches'sz and S1. The broadcast oscillator winding 56 is connected to the plate and cathode electrodes of the oscillator tube V1 through the agency of switches S5 and snrespectiv-ely. The grid of the oscillator tube is connected to the grounded end of the winding 56 by way of the condenser 21, conductor 26, winding 25, and resistor 51, the latter having a low resistance of say 10 ohms to reduce any possible tendency toward parasitic oscillations. The impedance of the winding is negligible at broadcast frequencies-and, hence, its presence in the oscillator grid return circuit can be ignored during AM operation.

Injection of the oscillator signal into the first detector circuit may be secured, during AM operation, by connecting the cathode of V2 to an intermediate point on the oscillator tank winding 56. This connection includes the cathode bias resistor 58, the lead 59, the lead 60, the switch S1 and the lead 6| The I. F. coupling transformer 30, which in the FM position of the switches Se and S9 (shown) is conditioned to transfer I. F. signals of the order of say 4.3 megacycles, canbe conditioned to transfer signals of the intermediate frequency commonly employed in AM receivers (for example 455 kilocycles). Thus, it will be seen that in the AM position of the switches, the I. F. by-pass condenser is shunted across the tuning condenser 33, thereby lowering the resonant frequency of the primary circuit which now consists essentially of the winding 31 and the fixed condenser 32. Similarly, when the switch S9 is moved to its AM position, the short circuit around the winding 39 is removed, and applied instead to the trimmer condenser 38, thus leaving a resonant secondary circuit tuned to 455 kilocycles and comprising the windings 36 and 39 and the condenser 31. As has already been indi cated, and as will be explained in greater detail hereinafter, the transformer 40 is adapted to transfer signals of either the AM or FM intermediate frequencies without switching.

Returning now to the detector and AVC cir-- cuits associated with the vacuum tube V5, it is pointed out that in the particular circuit of Fig. 5-, there are provided two sources of AVG voltage, namely, that at the junction of the resistor l4 and condenser l5, as already described in connection with Fig. 3, and that provided at the lefthand end of the resistor 62 whose other end is connected to the junction of the AVG voltage di-, viding resistors 63 and 64. The latter may be of the order of two megohms each, and adapted to provide a second AVC voltage whose magnitude is say one-half of that available at the junction of resistors l4 and 63. The condenser 65 may be connected between the upper end of resistor 64 and ground to provide filtering for the second AVC source. The control voltage from this source is applied to the gain control electrode of the first'I. F. amplifier V3 by way of the resistor 62, the conductor 66 and the coils 39 and 36. It will be seen that this AVC voltage is applied to V3 during both AM and FM operation ofthe receiver.

Full AVC voltage, on the other hand, is applied only to the control grid of the first detector tube V2, and this only during AM operation. The full AVC circuit may be traced from the junction of the elements l4 and I5, via conductor 61, switch S10 (AM position), conductor 68, and resistor 69 to the grid of V2. During FM operation, this AVC circuit is broken by the switch S10 and the lower end of the grid resistor 69 is connected to ground through the conductor 68,

grid bias under these conditions being supplied solely by'the cathode resistor 58.

Reference is now made to the novel intermediate frequency coupling network 40 shown schematically in Fig. 5 and illustrated in Fig. 6. This transformer comprises three windings 10, H, and 12, spaced along a tubular form 13, or

other suitable support. The windings 10. II. and

72 may be of the universal type, and in one embodiment of the invention had inductances of 55, 48, and 1000 microhenries, respectively. As is customary, the form 13 may be fixed to an insulating support 14, the latter carrying the various trimmer condensers employed in tuning the several windings. The coils l and H are tuned, by means of trimmer condensers connected thereacross, to resonate at the intermediate frequency employed for FM reception, commonly 4.3 megacycles. These coils are preferably overcoupled and damped sufficiently to provide a relatively flat response over the deviation range of the intermediate frequency. In the embodiment of Fig. 5, only the primary coil is damped, this being done by the connection thereacross of a suitable damping resistor of say 50,000 ohms. The winding 12, as will be explained hereinafter, is shunted by a relatively high capacity 16 which, at 4.3 megacycles (or in general, at the FM intermediatefrequency), by-passes the lower end of coil 12 to ground. Thus, at 4.3 megacycles, the.

transformer 40 may be regarded as a damped double-tuned transformer comprising only two windings, the primary winding 10 and the secondary winding H. j

v The winding 12 is tuned to resonate at the intermediate frequency employed in AM reception, for example, 455 kilocycles. The constants of the resonant circuit 12, 16, should be so selected that in addition to providing resonance at 455 kilocycles, thecapacity 16 will be large enough to act as a by-pass to currents having frequencies of the order of 4.3 megacycles. During 455 kc. operation, the winding 70 functions as an untuned primary. Preferably, the windings 10 and 12 are tightly coupled. Since the impedance of the high frequency winding II is negligible as compared to the impedance of the resonant circuit I2, 16, at 455 kilocycles, i. e., at the AM I. F., the transformer 40 may be regarded during AM operation as comprising only an untuned primary winding 10 and a tuned secondary winding 12 tightly coupled thereto. a From the foregoing, it will be seen that the transformer 40 provides a switchless, 4-terminal network' capable of functioning either as a wide-band double-tuned transformer at a relatively high intermediate frequency, or as a narraw-band single-tuned transformer at a relatively low intermediate frequency.

i It is preferred, in accordance with one of the featuresof this invention, that the receiver be provided with a plurality of stages of intermediate frequency amplification, at least one stage of WhlChjS operative, without switching, at either theAMorFMintermediate frequencies, and at leastone other stage of which is highly selective as between'the FM and the AM intermediate fre quency andirequires switching'from one. to the other. The radio receiver shown in Fig. 5 is 11- lustrativei'f f this preferred arrangement, Thus the transformer 40, as has already been explained in detail, is capable of transferring efficiently and effectively signals of either the AM or the 'FM intermediate frequencies ('e. g. 455 kc. or 4.3 mc. respectively),"without it being neces sary to switch any of its circuits to so adapt it. on the other hand the transformer 30 is responsive only tothe intermediate frequency to which itis adjusted and can beadapted torespond in a desired manner to the other intermediate frequency only by switching certain of its circuits, (ii

as for example by means of the switches Sa and S9 already described. W e

; Obviously if all intermediate frequency transformers were of the order of the unswitched variety shown at in Fig.5, the switching operations of the receiver could be still further simplified. However it has beenfound that it is highly desirable to have some selection, in the intermediate frequency portion of the receiver, as between the several intermediate frequencies to which the detector circuit is responsive. This is because in practice it is difficult, and usually impractical,

to designfa radio frequency and first detector circuit which will not, under certain conditions, contain in the output thereof signals of both the AM and FM intermediate frequencies. Assume, for example, that the receiver. isadjusted to receive an] FM signal having a nominal frequency of49.5 mo. Withthe localoscillator V1" adjusted to providea. heterodyne, signal of 45.2 mc. the desired intermediate frequency (difference frequency) can be obtained from the first detector V2, in this case 43 me. However if a strong FM signal from a nearby station having a nominal frequency of 45.7 me. should succeedin arriving at the input of the first detector tube Vzthere would appear, in the output of first detector v z, a spurious intermediate frequency signal having a frequency of 45.7-45.2 me. or 500.kc.- Since this spurious signal is frequency modulated to an extent equal to that of the undesired 45.7 mo. signal this spurious signal will spring betweenlimits of 400 and 600 kc., and hence will contain components in the 455 kc. intermediate frequency band. Hence unless means are provided in the intermediate frequency amplifier to discriminate againstthe undesired 455 kc. components during FM reception distorted detection will occur in the second detector Va. This is because in addition to the desired FMsignal being applied in balancedmanner to the diodes D1 and D2 there willalso be appliedgan unbalanced 455 kc. signal to the diode D2. In addition to the application of the vundesiredor spurious signal to D2 which is in itself v nidesirable, the balanced operation of the diodes at 43 n cfwiilbe interfered with, and the overall operation of the system be correspondingly impaired. g

Likewise, without due regard to the precautions observed in the receiver of 5, difficulties may be experienced when tuning stations in the broadcast band.- These diff culties maybe caused by a harmonic of the oscillator fallingin'the FM intermediate frequency band; and applying a spurious signal to the; diodes D1 and D2; ,Assu ne, for ex ample, that the receiver be adjusted to receive an AM signal having a frequency of 980 kc. This signal will be mixed, in the first detector V2, with an oscillator signal of 1435 kc.t o produce themdesired intermediate frequency of 455 kc, Now the third harmonic of the oscillator will-be 4.305 mc.,

; and this signalflies well within the band-pass characteristic of the 4.3 mc. intermediate frequency system. Consequently if means are not providedto reject or todiscrnninate against this signal both diodesD and; D2 will be subjeqtedto a strong unwanted 4.305 ,mc.,signal during recep-. tion of the desired 980 kc.AMsignal. l

In accordance with the inventiontheabovedescribed spurious signal difficulties maybeavoid ed by employing atfieast one switched selective circuit (in combination with theswitchlessucircuits) between the f rst and second -detectors. c a w q d sele e-.c cuitis,excmn ifiedby fi r s orm r .Q:whose,operationand circuit details have already been described; -:When

nals in the 4.3 mc. band. a i

" Preferably the-switched selective system is inserted in an early stage of the I. F. amplifier system, and in the preferred embodiment it forms the coupling means between the first detector V2 and the first I. F. amplifier tube V: as illustrated in Fig.5 of the drawings. This arrangement has been found most satisfactory since it is desirable to reject the'spurious signals at a relatively low level, rather than after they have been amplified. Referring to the invention generally, there is provided a radio receiver having a minimum of tubes andparts'; which is capableof receiving either amplitudeor-frequency-modulated waves with little orno more circuit switching than would normally be necessary in'switching from one AM band to another. There is provided an intermediate frequency transformer and a second detector input transformer operable On either of 7 two widely different intermediate frequencies without anyswitching whatever, and, likewise, there is provided a detector and AVC system operable without switchingon either AM or FM transmissions. Similarly, the I. F. filter, volume control, tone control, and detector output circuits are operable without change on either kind of signal, and these desirable features have been achieved without it being necessary to increase the number of tubes in the receiver over what would be require'd'either in a simple AM receiver ora'simpIeFMreceiVen Althoug-hthe invention has been described with particular referenceto radio receivers of the superheterodyne type and to the embodiments illustrated in-the drawings, it will be understood that the-invention is capable of general application and adaptedto other'forms of physical expression, and is; therefore, not to be limited to the specific disclosure; but only to the scope of the appended ,g1aim f} V I claim: r -1-.-"In-a-r'adio receiver of the class described, a frequency'detector-of the balanced variety in cludinga pa'irofvacuumtube devices, a pair of load impedances connected to said devices, means including a split frequency-discriminating network for supplying a received frequency-modulated signal to-said tubes," an input circuit adapted to provide a received amplitude-modulated signal"across 1 its output terminals, fixed means for connecting said terminals in series with apart of said frequency discriminatingnetworkand a predetermined one only of said vacuum tube devices'e' an audio frequency amplifier; fixed con'nections M for supplying detected signals from both of said load impedances to said ampli fier during theire ception of frequency-modulated signalsfandfixed connections for supplying detected signals from said predetermined one of said load-'impedances to said amplifier during the reception of amplitude-modulated signals.-

2. Ina radio receiver of the superheterodyne type having azfr'equency-converter and an'intermediate frequency amplifier, selective means re-' sponsive to an amplitude-modulated signal in one frequency band forestablishing an amplitudemodulated intermediatefrequency'signal of pre determined frequency, selective means responsive to a frequency-modulated signal in another irequency band for establishing a frequency-modulated intermediate frequency 'signal of substantially different frequency, a pair of detector elements, a discriminator-type coupling circuit operative at said different frequency and connected between the outputterminals of said intermediate frequency amplifier and said detector elements, a second coupling circuitoperative at said predetermined frequency and connected between the output circuit'of'said intermediate frequency amplifier and a predetermined one only of'said detector elements, the impedance v'ersus-frequency characteristic oi -each of said coupling circuits being such thattheyhave substantially no effect on the detection process for frequencies other than at the said intermediate frequencies at which they are operative,-a'nd an audio frequency output circuit having fixed connections to saiddetector elements for deriving an audio frequency output from both detector elements during the reception of frequency modulated signals, but only from said predetermined detector element during the reception of amplitude-modulatedsignals.

3. In a radio receiver of the superheterodyne type, a frequency converter system; radio frequency circuits for supplying either amplitudemodulated or frequency-modulated signals to said converter system, said converter system ineluding a plurality of selectable circuits for conditioning said converter to supply a first intermediate frequencysignalin response to frequencymodulated signals, and to supply a different intermediate frequency signal in response to amplitude-modulated signals,a detector circuit including a pair of detector elements, means includingatleast one circuit tuned to said first intermediate frequency for supplying said frequency-modulated intermediate frequency signal to both ofsaid detector elements in substantially ,balanced ;re1ation,. meanscomprising a circuit tuned to'said different intermediate frequency and operatively. connected to only one. of saiddetector elements :forappl-ying; said; amplitudemodulated intermediate I frequency .signal thereto, and an output circuit having fixed connections, to said detector elements forderiving a te te na ro .b t e t duri the ece t on; of w u n yem d tedr s s and for deriving a detected, signal from said one e1 eent during the reception of amplitudemodulated signals.

I f1. In a combination amplitude and fre quency-modulation radio receiver, a switchlss detector circuit, comprising a pair of detector elements, an intermediate frequency t ansformer opr'ativeat one frequency and having one tuned secondary winding and two", untuned secondary windings ,a connection between sneer said: tuned windings and bile 'Qffsaid detector elements; a connection between the other ,of :said untuned windings andthe other 'ofjsaid detector elements, means" for"conne cjt1 ng saidl l'tuned windingin .a circuit which v is common "to both of said" detector elements, a second intermediate frequency'transformerpperative at aflsubstantially different frequency and having a' tuned secondary winding, said last-mentioned 1 tunedf winding being connected ina circuit which is' common to only one of said detector elements and'its associated un tuned winding; all

of the aforesaid connections tionedtransformer and for alternatively supply eing free of any'associated' switching mean's, '-=and=' means for supplying an" amplitude modulated signal to said last-mentype] a frequency convertersystem, radio frequency circuits for supplyng ether ampltu'demodulated or frequency-modulated signals to.

said converter system, said converter-system in-.

eluding 'a plurality of selectable circuits for conditioning said: converter to supply one interme diate'frequency signal of -a nominal center fre-' quency in response to frequency-modulated'sig nals, and to supply a different intermediate frequency signal of a substantially different fre-" quencyin response to amplitude-modulated signals, aswitchless fixed-circuit inter-mediate frequency transformer having a plurality of windings tuned to said'different frequencies for trans fje'rring frequency-modulated signals at said one intermediate frequency and amplitude-modulated signals-at said different intermediate frequency, and a switchless fixed-circuit detector operative at said one intermediate frequency to detect frequency-modulated signals and opera tive-at said different intermediate frequency todetectamplitude-modulated signals.

"T6. In a radio receiver of the superheterodyne type, a frequency converter system, radio fre quency'circuits for supplying either amplitudemodulated-or frequency-modulated signals to said converter system, said converter system including a plurality of selectable circuits forconditioning said converter to supply a first intermediate frequency signal in response to ire-- quency-modulated signals, and to supply a substantially differentintermediate frequency signal in response to amplitude-modulated signals, a switchless fixed-circuit intermediate frequency transformer having a plurality of windings tuned to different frequencies for transferring frequency-modulated signals at said first intermediate frequency and amplitude-modulated signals at said difierent intermediate frequency, and a switchless fixed-circuit detector comprising a plurality of windings tuned to different frequencies corresponding to said intermediate frequencies, a pair of diodes and a load circuit therefor, said diodes being connected to said windings to provide balanced detection of frequency-modulated signals and single-diode detection of amplitudemodulated signals.

'7. In a combination amplitudeand frequencymodulation radio reeciver, a detector system including a transformer unit, said unit comprising a plurality of coils arranged in two groups and having a common support means, one of said groups comprising a pair of coils tuned to a predetermined intermediate frequency adapted for the transmission of amplitude-modulated signals, a second group of coils adapted for the transmission of frequency-modulated signals comprising a coil tuned to a higher intermediate frequency and a pair of untuned coils coupled tightly and positioned symmetrically thereto, another coil tuned to said higher intermediate frequency and positioned at a substantial distance from the first mentioned coil of said second group to insure loose coupling therebetween, and a connection between said untuned windings including one of the coils of said first group.

8. In a combination amplitudeand frequencymodulation radio receiver of the superheterodyne type, an intermediate frequency transformer unit adapted to function without switching at either of two substantially different intermediate frequencies, comprising a single primary winding of relatively low inductance tuned to a-relatively high intermediate frequency adapted for the transmission of frequency-modulated signals, a first secondary winding of relatively low induct-' ance tuned to said relatively high intermediate frequency, a second secondary winding of relatively high inductancepositionedclosely adj acent to said primary winding, said second secondary winding being tuned to a relatively low intermediate frequency adaptedfor the transmission ofamplitude-modulated signals, a pairof outputterminals,-and means for connecting said tuned secondary 'windings in series 'therebetween. v 9. In a combination amplitudeand frequency modulation radio receiver of the superheterodyne type, an intermediate-frequency transformer unit adapted to function without switching at either of-two substantially different intermediate fre quencies, comprising a single primary winding of relatively low inductance tuned to a relatively high intermediate frequency adapted for the transmission of frequency modulated.signals, a first secondary winding of relatively low inductance tuned to said relatively high intermediate frequency and 'overcoupled to said primary winding to afford wide-band transfer of said frequency-modulated signals, a second secondary winding of relatively high inductance tuned to a relatively low intermediate frequency adapted for the transmission of amplitude-modulated signals, said last-mentioned winding being tightlycoupled to said primarywinding, said primary winding functioning substantially as an untuned winding at said lower-intermediate frequency thereby to afford relatively narrow-band transfer of said amplitude-modulated signals, a pair of output terminals, and means for connecting said tuned secondary windings in series therebetween.

10. A radio receiver as claimed in claim 9, wherein said intermediate frequency transformer is characterized in the provision of means for damping at least one of the low inductance windings.

11. A radio receiver as claimed in claim 8, wherein said intermediate frequency transformer is characterized in that the tuning capacity associated with the said high-inductance secondary winding is relatively large, thereby effectively to by-pass said winding for currents of said relatively high frequency.

12. In aradioreceivenadetector circuit adapted to operate selectively and without switching upon signals of predetermined and substantially different characteristics, comprising a pair of diodes, a pair of load impedances therefor, a signal input circuit, a discriminator-type input circuit, means for supplying an amplitude-modulated signal of predetermined carrier frequency to said signal input circuit, means for supplying a frequency-modulated signal of substantially different carrier frequency to said discriminator-type input circuit, fixed connections for operatively interconnecting said discriminator-type input circuit, said diodes and said load inpedances to function as a frequency detector circuit during the reception of frequency-modulated signals, and fixed connections operatively interconnecting said signal input circuit, a predetermined one of said diodes and a predetermined one of said load impedances to function as an amplitude-modulation detector circuit during the reception of amplitude-modulated signals.

13. In a radio receiver of the class described, a combination switchless frequencyand ampliing fixed connections-t the other: of said devices,

said load impedances, saidspace discharge devices. and said frequ'ency-discriminating. means. being interconnected to provide balanced detection; of frequency-modulated signals; means for deriving a signal from both of said load impedances during thereception. of frequency-modulated signals, and means for deriving a signal from said .first load impedance. during the reception of amplitude-modulated:signals. V I

,14. A detector operative to convert both frequency-modulated carrier waves and amplitude:- modulated carrier waves. into audio-frequency; waves, said detector comprising. two rectifiers, afirst transformer means for transmitting, fre-. quency-modulated waves to said rectifiers differentially, said first transformer-means including a primary and. two secondary windings, said secondary windings being, connected in series and to said rectifiers individually, phase-shifting means operative in response to frequency variations of the. frequency-modulated wave for correspondingly unbalancing the-voltage applied to? said. rectifiers, and; a second. transformer for, transmitting amplitude-modulated waves into. arectifying circuit including atleast. one. of' said rectifiers said secondv transformer being, tuned to the frequency of the amplitude modulated.

wave and. including. a secondary winding interposed conductively between said first-mentioned secondary windings and in series therewith, said last mentioned secondary being characterized: by

low impedance to. the frequency-modulated.

fluency-modulated waves to said rectifiers dife ferentially, said first transformer means include ing a primary and two secondary windings, said secondary windings. being connected in series and to said rectifiers individually, phase-shifting, means operative in response to frequency variations of the frequency-modulated wave for correspondingly unbalancing the voltage applied to said rectifiers, and a second transformer for transmitting amplitude-modulated waves into a rectifying circuit including at least one. of said rectifiers, said second transformer being tuned to the. frequency of the amplitude-modulated wave and including a secondary winding in parallel with. a tuning condenser, said last-mentioned secondary winding being interposed conductively between said first-menti0ned secondary windings. and in series therewith.

CLAUDIUS T. MCCOY. 

